This invention relates to a method for the degradation of harmful substances, such as pollutants or contaminants, in water by means of compounds which form hydroxyl radicals, especially hydrogen peroxide, under UV irradiation. It is particularly preferred that the UV radiation is polychromatic radiation having a wavelength in the range of approximately 185 to 400 nm. Furthermore, the reaction may take place in a continuous flow reactor, through which the polluted or contaminated water flows, along with a sufficient amount of hydrogen peroxide.
U.S. Pat. No. 4,012,321; U.S. Pat. No. 4,446,029; German Patent Appl. No. 38 36 850 A; and D. Otto in "Wasserkalender," 1989 pp 134-158 show that the oxidation action of hydrogen peroxide can be vigorously increased if it is irradiated in an aqueous solution with monochromatic or polychromatic ultraviolet radiation. All of these references are entirely incorporated herein by reference. As a result of the UV irradiation, hydrogen peroxide decomposes into hydroxyl radicals and the harmful substances contained in the water are degraded by this strong oxidation agent. This treatment method is suitable for the degradation of families of compounds (classes of substances) which are chemically very different, such as hydrocarbons, alcohols, ethers, acids, aldehydes, ketones, amino compounds, halogen compounds, and cyanides. Typically, mercury low-pressure radiators are used as a radiation source for monochromatic radiation having wavelengths of, e.g., 185, 254 and 265 nm. Medium or high-pressure radiators are used for producing polychromatic radiation with a wavelength spectrum in the range of, e.g., 185 to 400 nm. To the extent it is desired, the oxidation action of the H.sub.2 O.sub.2 /UV irradiation system can be further increased by means of the addition of transitional metal ions, especially iron ions. This is described in U.S. Pat. No. 5,043,080, which patent is entirely incorporated by reference.
The treatment of water containing harmful substances with H.sub.2 O.sub.2 /UV radiation takes place on an industrial scale in continuous reactors. In the case of a low load of harmful substances, or in the case of readily degradable substances, a single passage of the water compounded with hydrogen peroxide through the irradiated reactor can be sufficient. A high load of harmful substances requires a process with reflux or recycle. It is known that the penetration depth of the radiation into the solution to be treated is only a few millimeters to a few centimeters. The penetration depth is a function of the wavelength of the radiation and the concentration of hydrogen peroxide and harmful substances in the water.
It is generally believed that, in order to achieve a satisfactory action, taking into consideration the indicated absorption behavior, the layers of liquid close to the radiator would have to be constantly renewed, and thus, the entire amount of liquid would have to be exposed to the short-wave radiation. To this end, the solution to be treated is pumped in a continuous manner at high speed through one or several continuous UV reactors connected in series. Typically, these UV reactors are tubular shaped. In the method of German Patent Appl. No. 38 36 850 A1, for example, reactors with a total layer thickness of irradiated solution of 1.65 to 12.5 cm are used, and the flow speed in the smallest section is adjusted to at least 0.2 m/sec. In the method of European Patent Appl. No. 0,436,922 A2 (which is also entirely incorporated herein by reference), tubular reactors are arranged around the radiation source. These tubular reactors have a radial layer thickness in the range of 0.1 to 50 mm.
In commercial tubular UV reactors, their specific volume is usually in the range of 1 to 10 liters per kilowatt (kW) of the electric power wattage of the radiator(s). The liquid flowthrough per hour should always be as high as possible, according to the data of the manufacturer, in order to achieve an ideal intermixing. Generally, the flowthrough rate is approximately 20 to 50 times the volume of the UV reactor.
A disadvantage of the previously known methods and commercial reactors is the requirement of having to use technically expensive and complicated UV reactors. In addition, the systems must be designed in such a manner that they can be operated with a high flowthrough rate, which further increases the necessary volume of capital. There was also interest in improving the rate of degradation of the harmful substances in water at a given performance (i.e., power, wattage) of the UV radiator (s) and with as low an outlay for equipment as possible.